Electrodeposition as a coating application method involves deposition of a film-forming composition onto a conductive substrate under the influence of an applied electrical potential. Electrodeposition has become increasingly important in the coatings industry because, by comparison with non-electrophoretic coating means, electrodeposition offers increased paint utilization, improved corrosion protection and low environmental contamination.
Initially, electrodeposition was conducted with the workpiece being coated serving as the anode. This was familiarly referred to as anionic electrodeposition. However, in 1972, cationic electrodeposition was introduced commercially. Since that time, cationic electrodeposition has steadily gained in popularity and today is by far the most prevalent method of electrodeposition. Throughout the world, more than 80 percent of all motor vehicles produced are given a primer coating by cationic electrodeposition.
Typically, electrodepositable coatings comprise an electrodepositable film-forming polymer and a curing agent therefor, in combination with, inter alia, various pigments. Lead-containing pigments such as lead silica chromate, basic lead silicate, lead chromate and lead sulfate are often used in electrodepositable coatings because they impart excellent corrosion resistance to the electrocoated article.
The acid used in cationic electrodeposition baths often solubilizes a portion of the lead pigment, thereby forming lead salts which are soluble in the aqueous phase of the electrodeposition bath. The lead ion is conveniently reduced to elemental lead in the presence of "mild steel", such as, the non-stainless steel pipes and equipment typically used in electrodeposition equipment. The elemental lead "plates out" or deposits on the surface of the mild steel surfaces which are in constant contact with the acidic electrodeposition bath compositions, forming a passivation or protective coating thereon. This lead passivation layer retards further oxidation of the mild steel surfaces by the acidic bath. Unfortunately, however, a portion of these lead salts often finds its way into the ultrafiltrate of the bath, thus necessitating the removal and subsequent disposal of metallic lead and/or ionic or organic lead-containing materials.
In recent years, due to environmental concerns, the use of lead-free coatings has been mandated. Although surface coatings of excellent quality can be achieved by means of cationic electrodeposition of lead-free coatings, the removal of corrosion inhibitive lead pigments can result in poor corrosion resistance of these coatings, particularly when applied to untreated steel substrates. Moreover, in lead-free electrodeposition baths, the mild steel pipes and equipment are left unprotected and are subjected to corrosive attack by the acidic cationic electrodeposition bath composition.
The use of phosphoric acid in electrodeposition baths for reducing corrosivity of electrodeposition baths is well known in the art. Also, U.S. Pat. No. 4,218,296 discloses the use of C.sub.1 -C.sub.4 fatty acid anions in conjunction with H.sub.2 PO.sub.4 -anions as counterions to balance the ionized cationogenic sites of a cathodically electrodepositable resin, which suppresses both iron staining of the coating and corrosion of iron and steel electrodeposition equipment. The addition of phosphoric acid to electrodeposition baths at levels high enough to effectuate such improvements, however, typically results in poor appearance, i.e., pinholing, of the resultant coating, believed to be caused by gas evolution at the cathode.
The addition of other phosphorous-based compounds, particularly organic phosphorous-based acids and esters, to improve corrosion resistance of the electrocoated article and reduce the corrosivity of lead-free cationic electrodeposition baths is not known. It therefore, would be desirable to provide an electrodeposition bath composition, particularly a lead-free electrodeposition bath, which provides improved corrosion resistance of the electrocoated article as well as a reduction in corrosivity of the bath equivalent to that of phosphoric acid, without adversely affecting appearance of the resultant coating.